Streptococcus pneumoniae is an important cause of otitis media, meningitis, bacteremia and pneumonia. Despite the use of antibiotics and vaccines, the prevalence of pneumococcal infections has declined little over the last twenty-five years.
It is generally accepted that immunity to Streptococcus pneumoniae can be mediated by specific antibodies against the polysaccharide capsule of the pneumococcus. However, neonates and young children fail to make an immune response against polysaccharide antigens and can have repeated infections involving the same capsular serotype.
One approach to immunizing infants against a number of encapsulated bacteria is to conjugate the capsular polysaccharide antigens to protein to make them immunogenic. This approach has been successful, for example, with Haemophilus influenzae b (see U.S. Pat. No. 4,496,538 to Gordon and U.S. Pat. No. 4,673,574 to Anderson). However, there are over eighty known capsular serotypes of S. pneumoniae of which twenty-three account for most of the disease. For a pneumococcal polysaccharide-protein conjugate to be successful, the capsular types responsible for most pneumococcal infections would have to be made adequately immunogenic. This approach may be difficult, because the twenty-three polysaccharides included in the presently-available vaccine are not all adequately immunogenic, even in adults.
An alternative approach for protecting children, and also the elderly, from pneumococcal infection would be to identify protein antigens that could elicit protective immune responses. Such proteins may serve as a vaccine by themselves, may be used in conjunction with successful polysaccharide-protein conjugates, or as carriers for polysaccharides.
In McDaniel et al (I), J.Exp.Med. 160:386-397, 1984, there is described the production of hybridoma antibodies that recognize cell surface polypeptide(s) on S. pneumoniae and protection of mice from infection with certain strains of encapsulated pneumococci by such antibodies. This surface protein antigen has been termed "pneumococcal surface protein A" or PspA for short.
In McDaniel et al (II), Microbial Pathogenesis 1:519-531, 1986, there are described studies on the characterization of the PspA. Considerable diversity in the PspA molecule in different strains was found, as were differences in the epitopes recognized by different antibodies.
In McDaniel et al (III), J.Exp.Med. 165:381-394, 1987, there is disclosed that immunization of X-linked immunodeficient (XID) mice with non-encapsulated pneumococci expressing PspA, but not isogenic pneumococci lacking PspA, protects mice from subsequent fatal infection with pneumococci.
In McDaniel et al (IV), Infect. Immun., 59:222-228, 1991, there is described immunization of mice with a recombinant full length fragment of PspA that is able to elicit protection against pneumococcal strains of capsular types 6A and 3.
In Crain et al, Infect.Immun., 56:3293-3299, 1990, there is described a rabbit antiserum that detects PspA in 100% (n=95) of clinical and laboratory isolates of strains of S. pneumoniae. When reacted with seven monoclonal antibodies to PspA, fifty-seven S. pneumoniae isolates exhibited thirty-one different patterns of reactivity.
The PspA protein type is independent of capsular type. It would seem that genetic mutation or exchange in the environment has allowed for the development of a large pool of strains which are highly diverse with respect to capsule, PspA, and possibly other molecules with variable structures. Variability of PspA's from different strains also is evident in their molecular weights, which range from 67 to 99 kD. The observed differences are stably inherited and are not the result of protein degradation.
Immunization with a partially purified PspA from a recombinant .lambda. gt11 clone, elicited protection against challenge with several S. pneumoniae strains representing different capsular and PspA types, as described in McDaniel et al (IV), Infect. Immun. 59:222-228, 1991. Although clones expressing PspA were constructed according to that paper, the product was insoluble and isolation from cell fragments following lysis was not possible.
While the protein is variable in structure between different pneumococcal strains, numerous cross-reactions exist between all PspA's, suggesting that sufficient common epitopes may be present to allow a single PspA or at least a small number of PspA's to elicit protection against a large number of S. pneumoniae strains.
In addition to the published literature specifically referred to above, the inventors, in conjunction with co-workers, have published further details concerning PspA's, as follows:
1. Abstracts of 89th Annual Meeting of the American Society for Microbiology, p. 125, item D-257, May 1989; PA1 2. Abstracts of 90th Annual Meeting of the American Society for Microbiology, p. 98, item D-106, May 1990; PA1 3. Abstracts of 3rd International ASM Conference on Streptococcal Genetics, p. 11, item 12, June 1990; PA1 4. Talkington et al, Infect. Immun. 59:1285-1289, 1991; PA1 5. Yother et al, J. Bacteriol. 174:601-609, 1992; and PA1 6. Yother et al, J. Bacteriol. 174:610-618, 1992. The latter three publications occurred after the filing of the aforesaid U.S. Ser. No. 656,773.
In the specification which follows and the drawings accompanying the same, there are utilized certain accepted abbreviations with respect to the amino acids represented thereby. The following Table I identifies those abbreviations:
TABLE I ______________________________________ AMINO ACID ABBREVIATIONS ______________________________________ A = Ala = Alanine M = Met = Methionine C = Cys = Cysteine N = Asn = Asparagine D = Asp = Aspartic Acid P = Pro = Proline E = Glu = Glutamic Acid Q = Gln = Glutamine F = Phe = Phenylalanine R = Arg = Arginine G = Gly = Glycine S = Ser = Serine H = His = Histidine T = Thr = Threonine I = Ile = Isoleucine V = Val = Valine K = Lys = Lysine W = Try = Tryptophan L = Leu = Leucine Y = Tyr = Tyrosine ______________________________________